Drawn composite fiber, non-woven fabric, and method of producing drawn composite fiber

ABSTRACT

Embodiments relate to a drawn composite fiber having a low thermal shrinkage, and a high single yarn strength, a non-woven fabric using the same, and a method of producing the same. The drawn composite fiber has a fineness of 0.6 dtex or less, a ratio between the cross-sectional areas of a sheath material and a core material (sheath material/core material) of 50/50 to 10/90, and a single yarn elastic modulus of 70 cN/dtex or more. The drawn composite is obtained by melt-spinning and a drawing treatment of an undrawn fiber having a sheath-core structure, in which the core material includes a resin containing a crystalline propylene-based polymer and having a melt flow rate of 10 to 30 g/10 min at a load of 21.18 N at 230° C., and the sheath material includes a resin containing an olefinic polymer where the melting point is lower than that of the core material.

RELAYED APPLICATIONS

The present application is a National Phase of International Application No. PCT/JP2020/011925 filed Mar. 18, 2020, which claims priority to Japanese Application No. 2019-068001, filed Mar. 29, 2019.

TECHNICAL FIELD

The present invention relates to a drawn composite fiber having a sheath-core structure, a non-woven fabric, and a method of producing the drawn composite fiber. More specifically, the present invention relates to a drawn composite fiber having a thin fineness of 0.6 dtex or less, a method of producing the drawn composite fiber, and a non-woven fabric using the drawn composite fiber having the thin fineness.

BACKGROUND ART

Composite fibers with a sheath-core structure, formed using two olefinic resins having different characteristics, are utilized in various fields because of having a thermal adhesion property and excellent chemical resistance. For example, such composite fibers with a sheath-core structure can be produced by drawing treatment of undrawn fibers with a sheath-core structure, formed by melt-spinning.

It is demanded that functional non-woven fabrics used in various filter materials, separators for batteries, and the like are thin films and have a high mechanical strength. The thinner fineness and improved single yarn strength of raw material fibers in comparison with conventional ones are required for achieving such a non-woven fabric that is a thin film and has a high mechanical strength. Common examples of methods of increasing the single yarn strength and elastic modulus of drawn composite fibers include an increase in draw magnification. However, such an increase in draw magnification has problems of resulting in yarn breakage in drawing, the deterioration of non-woven fabric processability, caused by an increase in the thermal shrinkage of drawn fibers, and the deterioration of the appearance of a processed non-woven fabric.

Thus, technologies of producing drawn composite fibers having a high strength and a thin fineness by methods other than an increase in draw magnification have been conventionally proposed (see, for example, Patent Literatures 1 and 2). For example, in a composite fiber described in Patent Literature 1, the higher strength of the composite fiber is intended to be achieved by specifying the ratio between the weight-average molecular weights of a crystalline propylene-based polymer which is a core material and an olefinic polymer which is a sheath material, the melt flow rates (MFR) of the sheath material and the core material, and the like.

In a method of producing a composite fiber described in Patent Literature 2, the melt flow rate of a core material discharged from a spinneret is specified, and the ratio between the melt flow rate of the core material discharged from the spinneret and the melt flow rate of a sheath material discharged from the spinneret (=core material MFR/sheath material MFR) is specified, in order to obtain the composite fiber that has high strength and thin fineness.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Laid-Open No. 2007-107143

Patent Literature 2: International Publication No. WO 2015/012281

SUMMARY OF INVENTION Technical Problem

In production of a non-woven fabric, a raw material fiber having a suitable fineness is selected and used depending on intended characteristics such as a thickness, a basis weight, a filling rate, a pore diameter, and strength. In such a case, the non-woven fabric may be produced from one raw material fiber; however, an ultrafine fiber having a fineness of around 0.1 dtex and a thin fineness fiber having a fineness of around 0.2 to 0.6 dtex may be kneaded to obtain the non-woven fabric having two characteristics such as a fine pore diameter and a non-woven fabric strength. Enhancement of the physical properties such as a single yarn strength and an elastic modulus of both the ultrafine fiber and the thin-fineness fiber which are raw materials is required for improving the strength of such a non-woven fabric. In the above-described technology described in the Patent Literature 1, however, the composite fiber having a fineness of around 1 dtex is targeted, and, in addition, the obtained composite fiber has a high thermal shrinkage of 10% or more.

In contrast, in the production method described in Patent Literature 2, the drawn composite fiber having a single yarn strength of 5 cN/dtex or more, a Young's modulus of 50 cN/dtex or more, and a thermal shrinkage of 8% or less at 120° C. can be obtained. However, the technology targets an ultrafine composite fiber having a fineness of 0.3 dtex or less, and it is difficult to obtain the equivalent characteristics of a thin-fineness composite fiber that is thicker than the composite fiber. While further improvement in the physical properties of single yarn and a non-woven fabric is desired, there is a limit to the further improvement in the physical properties such as a single yarn strength and an elastic modulus even in the case of drawing at a high magnification in a drawing step in the production by the method disclosed in the conventional technology.

Thus, an objective of the present invention is to provide a drawn composite fiber having a fineness of 0.6 dtex or less, a low thermal shrinkage, and a high single yarn strength, a non-woven fabric, and a method of producing the drawn composite fiber.

Solution to Problem

A drawn composite fiber according to the present invention is a drawn composite fiber including a sheath-core structure in which a resin containing a crystalline propylene-based polymer as a main component is a core material, and a resin containing, as a main component, an olefinic polymer of which a melting point is lower than that of the core material is a sheath material, wherein the drawn composite fiber has a fineness of 0.6 dtex or less, a melt flow rate of the core material at a load of 21.18 N at 230° C. is 10 to 30 g/10 min, a ratio between cross-sectional areas of the sheath material and the core material (sheath material/core material) is 50/50 to 10/90, and the drawn composite fiber has a single yarn elastic modulus of 70 cN/dtex or more.

In the drawn composite fiber, a ratio between a melt flow rate of the core material at a load of 21.18 N at 230° C. and a melt flow rate of the sheath material at a load of 21.18 N at 230° C. (core material/sheath material) is, for example, 0.3 to 1.

A non-woven fabric according to the present invention is formed using the drawn composite fiber described above.

A method of producing a drawn composite fiber according to the present invention includes: a spinning step of obtaining, by melt-spinning, an undrawn fiber including a sheath-core structure in which a resin containing a crystalline propylene-based polymer as a main component is a core material, and a resin containing, as a main component, an olefinic polymer of which a melting point is lower than that of the core material is a sheath material; and a drawing step of obtaining a drawn composite fiber having a fineness of 0.6 dtex or less by drawing treatment of the undrawn fiber, wherein the undrawn fiber has a fineness of 4.0 dtex or less, and has a ratio between cross-sectional areas of the sheath material and the core material (sheath material/core material) of 50/50 to 10/90, the core material has a melt flow rate of 10 to 30 g/10 min at a load of 21.18 N at 230° C., and the spinning step and the drawing step are consecutively performed.

In the method of producing a drawn composite fiber, a ratio between a melt flow rate of the core material at a load of 21.18 N at 230° C. and a melt flow rate of the sheath material at a load of 21.18 N at 230° C. (core material/sheath material) may be set in a range of 0.3 to 1.

The draw magnification of the undrawn fiber in the drawing step is, for example, 2 to 7 times.

A value of a melt flow rate in the present invention is a value measured under conditions of a temperature of 230° C. and a load of 21.18 N according to A-method in JIS K7210, and the same applies in the following description unless otherwise specified.

Advantageous Effects of Invention

In accordance with the present invention, in a drawn composite fiber having a fineness of 0.6 dtex or less, a single yarn strength can be enhanced without increasing a thermal shrinkage.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view schematically illustrating an example of the cross-section structure of a drawn composite fiber of an embodiment of the present invention.

FIG. 2 is a flow chart illustrating a method of producing a drawn composite fiber of an embodiment of the present invention.

FIG. 3 is a schematic view illustrating a configuration example of an apparatus in the case of consecutively performing each step illustrated in FIG. 2.

FIGS. 4A and 4B are schematic views illustrating apparatus configurations in the case of separately performing each step illustrated in FIG. 2, FIG. 4A illustrates the spinning step, and FIG. 4B illustrates the drawing step.

DESCRIPTION OF EMBODIMENTS

Embodiments for carrying out the present invention will be described in detail below with reference to the accompanying drawings. The present invention is not limited to the embodiments described below FIG. 1 is a view schematically illustrating an example of the cross-section structure of a drawn composite fiber of the present embodiment. As illustrated in FIG. 1, a drawn composite fiber of the present embodiment is a sheath-core composite fiber including a core portion 1 and a sheath portion 2 formed in the periphery thereof, and has a fineness of 0.6 dtex or less, and preferably 0.2 to 0.6 dtex.

[Core Portion 1]

The core portion 1 contains a crystalline propylene-based polymer as a main component, and is formed of a resin having a melt flow rate (MFR) of 10 to 30 g/10 min at a load of 21.18 N at 230° C. (hereinafter referred to as “core material”). In a case in which the MFR of the core material is less than 10 g/10 min, the melt tension of the molten resin is prone to be higher, it is difficult to obtain an undrawn fiber having an intended fineness, and, in addition, drawing of an undrawn fiber at a high magnification tends to result in an increase in the frequency of occurrence of yarn breakage.

In a case in which the MFR of the core material is more than 30 g/10 min, the melt tension of the molten resin is lower, and therefore, the orientation crystallinity degree of an undrawn fiber is decreased, whereby it is impossible to sufficiently enhance the single yarn strength and elastic modulus of the drawn composite fiber, and it is difficult to obtain intended single yarn physical properties. The MFR of the core material is preferably set at 15 to 25 g/10 min, and the setting of the MFR in this range enables the strength of the drawn composite fiber to be expressed while decreasing the fineness of the undrawn fiber.

As the crystalline propylene-based polymer which is the main component of the core material, for example, an isotactic propylene homopolymer having crystallinity, an ethylene-propylene random copolymer having a low ethylene unit content, a propylene block copolymer including a homo portion including a propylene homopolymer and a copolymerization portion including an ethylene-propylene random copolymer having a relatively high ethylene unit content, in addition, a crystalline propylene-ethylene-α-olefin copolymer in which each homo portion or copolymerization portion in a propylene block copolymer includes a substance obtained by copolymerization of an α-olefin such as butene-1, or the like can be used, and isotactic polypropylene is particularly preferred from the viewpoint of drawability, fiber physical properties, and suppression of thermal shrinkage. These crystalline propylene-based polymers may be used singly, or in combination of two or more kinds thereof.

The core material can be blended with an additive such as a nucleating agent or an antioxidant at an appropriate rate. In a relationship with the resin containing the crystalline propylene-based polymer as the main component, the additive blended into the core material is preferably an additive which is melted together to develop an affinity, or an additive which is not completely melted and of which part adapts to the resin.

[Sheath Portion 2]

The sheath portion 2 is formed of a resin containing, as a main component, an olefinic polymer of which the melting point is lower than that of the core material (hereinafter referred to as “sheath material”). As the olefinic polymer which is the main component of the sheath material, for example, an ethylene polymer such as a high-density polyethylene, medium-density polyethylene, low-density polyethylene and a linear low-density polyethylene, a copolymer of propylene and another α-olefin, specifically, propylene-butene-1-random copolymer, propylene-ethylene-butene-1 random copolymer, or an amorphous propylene-based polymer such as soft polypropylene, poly 4-methylpentene-1, or the like can be used, and a high-density polyethylene is particularly preferred in view of fiber physical properties. These olefinic polymers may be used singly, or in combination of two or more kinds thereof.

The sheath material can be blended with an additive such as a nucleating agent or an antioxidant at an appropriate rate. In a relationship with the resin containing the olefinic polymer as the main component, the additive blended into the sheath material is preferably an additive which is melted together to develop an affinity, or an additive which is not completely melted and of which part adapts to the resin.

[Sheath-Core Ratio]

The drawn composite fiber of the present embodiment has a sheath-core ratio, i.e., an area ratio between the core portion 1 and the sheath portion 2 in a cross section (cross section perpendicular to lengthwise direction) (sheath material/core material) of 50/50 to 10/90. In a case in which the ratio of the core portion 1 in the cross section is less than 50%, the single yarn strength and elastic modulus of the drawn composite fiber are insufficient, and, in addition, a thermal shrinkage is also increased. In a case in which the ratio of the core portion 1 in the cross section is more than 90%, the sheath material contributing to thermal fusion is insufficient, and the strength of a processed product such as a non-woven fabric is decreased. In a case in which the ratio of the core portion 1 in the cross section is too high, a draw magnification is decreased, whereby yarn breakage is prone to occur, in the drawing step.

[Core Material MFR/Sheath Material MFR]

The drawn composite fiber of the present embodiment preferably has a ratio the MFR of the core material (pellet) at a load of 21.18 N at 230° C. and the MFR of the sheath material (pellet) at a load of 21.18 N at 230° C. (core material MFR/sheath material MFR) of 0.3 to 1. In a case in which core material MFR/sheath material MFR is less than 0.3, the melt tension of a molten resin is prone to be higher, and it may be impossible to produce an undrawn fiber having an intended fineness. In a case in which core material MFR/sheath material MFR is more than 1, the melt tension of the molten resin is too low, the single yarn strength and elastic modulus of the drawn composite fiber are decreased, and it may be impossible to obtain intended single yarn physical properties.

[Single Yarn Elastic Modulus]

The drawn composite fiber of the present embodiment has a single yarn elastic modulus of 70 cN/dtex or more. In a case in which the drawn composite fiber has a single yarn elastic modulus of less than 70 cN/dtex, the mechanical strength of a thin-film non-woven fabric is insufficient, and rupture or poor appearance is prone to occur, when the drawn composite fiber is processed into the thin-film non-woven fabric.

[Production Method]

A method of producing a drawn composite fiber of the present embodiment will now be described. FIG. 2 is a flow chart illustrating the method of producing a drawn composite fiber of the present embodiment, and FIG. 3 is a schematic view illustrating a configuration example of an apparatus in the case of consecutively performing each step illustrated in FIG. 2. As illustrated in FIG. 2, the spinning step (step S1) of obtaining an undrawn fiber having a sheath-core structure by melt-spinning, and the drawing step (step S2) of obtaining a drawn composite fiber by drawing treatment of the undrawn fiber are consecutively performed in the method of producing a drawn composite fiber of the present embodiment.

<Spinning Step S1>

In the spinning step S1, an undrawn fiber with a sheath-core structure having a fineness of 4.0 dtex or less, preferably 0.35 to 4.0 dtex and a sheath-core ratio (sheath material/core material) of 50/50 to 10/90 is melt-spun. In such a case, a resin containing a crystalline propylene-based polymer as a main component, and having a melt flow rate of 10 to 30 g/10 min at a load of 21.18 N at 230° C. is used in the core material, and a resin containing, as a main component, an olefinic polymer of which the melting point is lower than that of the core material is used in the sheath material. Moreover, core material MFR/sheath material MFR is preferably set in a range of 0.3 to 1 for the reason described above.

(Undrawn Fiber)

Like a drawn composite fiber, the sheath material/core material of an undrawn fiber is also set at 50/50 to 10/90 because the sheath-core ratio of the undrawn fiber is the sheath-core ratio of the drawn composite fiber. In a case in which the fineness of the undrawn fiber is set at 4.0 dtex or more, the enhancement of a draw magnification is required for setting the fineness of the drawn composite fiber at 0.6 dtex or less, yarn breakage is prone to occur in drawing, and the thermal shrinkage of the drawn fiber is prone to be deteriorated. Therefore, in the drawn composite fiber of the present embodiment, the fineness of the undrawn fiber is set at 4.0 dtex or less.

When a resin with an MFR of 10 to 30 g/10 min (at 230° C. and a test load of 21.18 N), used as the core material in the drawn composite fiber of the present embodiment, is allowed to be a molten resin, the resin is prone to result in a higher tension, and therefore, it is difficult to stably spin an undrawn fiber having a fineness of less than 0.35 dtex. Therefore, the fineness of the undrawn fiber is preferably set in a range of 0.35 to 4.0 dtex.

<Drawing Step S2>

In the drawing step S2, the drawn composite fiber having a fineness of 0.6 dtex or less, preferably 0.2 to 0.6 dtex, is obtained by drawing treatment of the undrawn fiber. In such a case, when the draw magnification is less than 2 times, the single yarn strength and elastic modulus of the obtained drawn composite fiber may be decreased, and intended single yarn physical properties may be prevented from being obtained. When the draw magnification is more than 7 times, a frequency at which yarn breakage occurs may be increased, and productivity may be deteriorated. Thus, the draw magnification in the drawing step S2 is preferably set at 2 to 7 times.

<Direct Spinning Drawing Method>

The drawn composite fiber of the present embodiment is produced by a direct spinning drawing method (spin-draw method) in which the spinning step S1 and the drawing step S2, described above are consecutively performed. For example, in the case of an apparatus illustrated in FIG. 3, an undrawn fiber 10 with a sheath-core structure, discharged from a spinneret 11 is introduced into a vapor drawing bath 13 through an introduction roller 12, and drawn at a predetermined magnification, and a drawn composite fiber 20 is then delivered by a delivery roller 14, and wound by a winder 15.

When a spinning step and a drawing step are separately and inconsecutively performed like a two-stage drawing method, it is difficult to draw an undrawn fiber having a thin fineness at a high magnification, and it is impossible to obtain a drawn composite fiber having an intended strength and an elastic modulus at a magnification at which the undrawn fiber can be drawn. In contrast, in the direct spinning drawing method (spin-draw method) in which the spinning step and the drawing step are consecutively performed, an undrawn fiber can be stably and immediately transferred to the drawing step, even an undrawn fiber with a thin fineness, which is easily cut due to drawing, can be drawn in the state of being homogeneous and easily stretched, and a drawn composite fiber with excellent single yarn physical properties is obtained. As a result, a drawn composite fiber having a fineness of 0.6 dtex or less, a high single yarn strength, a high single yarn elastic modulus, and a low thermal shrinkage can be produced from an undrawn fiber having a fineness of 4.0 dtex or less.

The drawn composite fiber produced by the method described above can be allowed to be in the form of a long-fiber filament used for a woven fabric through oil solution treatment and drying treatment. To be in a form used for a non-woven fabric, the drawn composite fiber may also be allowed to be a staple fiber through oil solution treatment, crimping processing treatment, and drying treatment subsequently to the drawing step. Further, the drawn composite fiber may also be cut into short fibers through or without through drying treatment after oil solution treatment, and allowed to be chopped fibers.

As described in detail above, the drawn composite fiber of the present embodiment has the MFR of the core material, the sheath-core ratio, and the single yarn elastic modulus, set in the specific ranges, and can therefore have a single yarn strength of 6 cN/dtex or more and a bundle thermal shrinkage at 120° C., reduced to 8% or less, despite having a thin fineness of 0.6 dtex. As described above, the drawn composite fiber of the present embodiment has a high strength and a low thermal shrinkage, and can be therefore preferably used in various applications for non-woven fabrics, and applications such as battery separators and filters. A thin-film non-woven fabric formed using the drawn composite fiber of the present embodiment has a high mechanical strength and suppressed thermal shrinkage in processing, and can therefore result in elimination of occurrence of poor processing, such as rupture, and poor appearance.

EXAMPLES

The effects of the present invention will be specifically described below with reference to Examples and Comparative Examples. In the examples, the drawn composite fibers of Examples and Comparative Examples were produced by a method described below, and the performance thereof was evaluated.

[Raw Materials]

(1) Core Material

A: Isotactic polypropylene “Y2005GP” manufactured by Prime Polymer Co., Ltd.

(MFR=20 g/10 min, Q value=4.7)

B: Isotactic polypropylene “Y2000GV” manufactured by Prime Polymer Co., Ltd.

(MFR=18 g/10 min, Q value=3.0)

C: Isotactic polypropylene “S119” manufactured by Prime Polymer Co., Ltd.

(MFR=60 g/10 min, Q value=2.8)

D: Isotactic polypropylene “S137L” manufactured by Prime Polymer Co., Ltd.

(MFR=30 g/10 min, Q value=3.2)

(2) Sheath Material

a: High-density polyethylene “S6932” manufactured by KEIYO POLYETHYLENE CO., LTD.

(MFR=40 g/10 min, Q value=5.1)

b: High-density polyethylene “J300” manufactured by Asahi Kasei Chemicals Corp.

(MFR=70 g/10 min, Q value=4.3)

[Evaluation/Measurement Methods]

(1) Fineness

The finenesses of an undrawn fiber and a drawn composite fiber were measured in conformity with JIS L1015.

(2) MFR

The MFR of each material pellet used in the core material and the sheath material was measured according to A-method in JIS K7210 under conditions of a test temperature of 230° C. and a test load of 21.18 N.

(3) Single Yarn Physical Properties of Drawn Composite Fiber

The single yarn strength and elastic modulus of a drawn composite fiber were measured by a method in conformity with JIS L1015.

(4) Bundle Physical Properties of Drawn Composite Fiber

The thermal shrinkage of a fiber bundle (bundle) was measured by a method in conformity with JIS L1015. In such a case, the number of filaments was set at 12018, heat treatment temperature was set at 120° C., and heat treatment time was set at 10 minutes.

Example 1

The spinning step and the drawing step were consecutively performed using the apparatus illustrated in FIG. 3, to produce a drawn composite fiber having a sheath-core structure.

(1) Spinning Step

An undrawn fiber with a sheath-core structure having a fineness of 1.88 dtex was produced by melt-spinning using a core material A and a sheath material a. In such a case, a sheath-core-type composite spinneret was used, and a sheath-core ratio (sheath material/core material) was set at 35/65. As spinning conditions, extruder cylinder temperature was set at 255° C., spinneret temperature was set at 270° C., and a spinning speed was set at 180 m/min.

(2) Drawing Step

The drawing step was performed subsequently to the spinning step. Specifically, the undrawn fiber 10 obtained in the spinning step was introduced into the introduction roller 12 at a speed of 180 m/min, the speed of the drawn fiber delivery roller 14 was increased, and the undrawn fiber 10 was drawn in the vapor drawing bath 13 with ordinary pressure vapor at 100° C.

As a result, the speed of the drawn fiber delivery roller 14 and a draw magnification, at which yarn breakage did not occur in the spinning step and the drawing step, and it was possible to perform industrially stable drawing, were 910 m/min and 5.10 times, respectively. Moreover, the fineness of the drawn composite fiber of Example 1 produced under such conditions was 0.4 dtex.

Example 2

An undrawn fiber having a fineness of 1.72 dtex was melt-spun by a method and under conditions similar to those in Example 1 except that a core material B was used instead of the core material A, and a sheath-core ratio (sheath material/core material) was set at 25/75, and the undrawn fiber was drawn by a method and under conditions similar to those in Example 1.

As a result, the speed of a drawn fiber delivery roller 14 and a draw magnification, at which yarn breakage did not occur in a spinning step and a drawing step, and it was possible to perform industrially stable drawing, were 841 m/min and 4.67 times, respectively. Moreover, the fineness of a drawn composite fiber of Example 2 produced under such conditions was 0.4 dtex.

Example 3

An undrawn fiber having a fineness of 1.60 dtex was melt-spun by a method and under conditions similar to those in Example 1 except that a sheath-core ratio (sheath material/core material) was set at 50/50, and the undrawn fiber was drawn by a method and under conditions similar to those in Example 1.

As a result, the speed of a drawn fiber delivery roller 14 and a draw magnification, at which yarn breakage did not occur in a spinning step and a drawing step, and it was possible to perform industrially stable drawing, were 781 m/min and 4.34 times, respectively. Moreover, the fineness of a drawn composite fiber of Example 3 produced under such conditions was 0.4 dtex.

Example 4

An undrawn fiber having a fineness of 0.80 dtex was melt-spun by a method and under conditions similar to those in Example 1 except that a core material D was used instead of the core material A, and a sheath-core ratio (sheath material/core material) was set at 50/50, and the undrawn fiber was drawn by a method and under conditions similar to those in Example 1.

As a result, the speed of a drawn fiber delivery roller 14 and a draw magnification, at which yarn breakage did not occur in a spinning step and a drawing step, and it was possible to perform industrially stable drawing, were 781 m/min and 4.34 times, respectively. Moreover, the fineness of a drawn composite fiber of Example 4 produced under such conditions was 0.2 dtex.

Example 5

An undrawn fiber having a fineness of 0.80 dtex was melt-spun by a method and under conditions similar to those in Example 1 except that the core material D and a sheath material b were used, and a sheath-core ratio (sheath material/core material) was set at 50/50, and the undrawn fiber was drawn by a method and under conditions similar to those in Example 1.

As a result, the speed of a drawn fiber delivery roller 14 and a draw magnification, at which yarn breakage did not occur in a spinning step and a drawing step, and it was possible to perform industrially stable drawing, were 781 m/min and 4.34 times, respectively. Moreover, the fineness of a drawn composite fiber of Example 5 produced under such conditions was 0.2 dtex.

Comparative Example 1

An undrawn fiber having a fineness of 1.60 dtex was melt-spun by a method and under conditions similar to those in Example 1 except that the core material C and the sheath material b were used, and a sheath-core ratio (sheath material/core material) was set at 50/50, and the undrawn fiber was drawn by a method and under conditions similar to those in Example 1.

As a result, the speed of a drawn fiber delivery roller 14 and a draw magnification, at which yarn breakage did not occur in a spinning step and a drawing step, and it was possible to perform industrially stable drawing, were 781 m/min and 4.34 times, respectively. Moreover, the fineness of a drawn composite fiber of Comparative Example 1 produced under such conditions was 0.4 dtex.

Comparative Example 2

An undrawn fiber having a fineness of 1.60 dtex was melt-spun by a method and under conditions similar to those in Example 1 except that a sheath-core ratio (sheath material/core material) was set at 60/40, and the undrawn fiber was drawn by a method and under conditions similar to those in Example 1.

As a result, the speed of a drawn fiber delivery roller 14 and a draw magnification, at which yarn breakage did not occur in a spinning step and a drawing step, and it was possible to perform industrially stable drawing, were 781 m/min and 4.34 times, respectively. Moreover, the fineness of a drawn composite fiber of Comparative Example 2 produced under such conditions was 0.4 dtex.

Comparative Example 3

A spinning step and a drawing step were inconsecutively performed using apparatuses illustrated in FIGS. 4A and 4B, to produce a drawn composite fiber having a sheath-core structure.

(1) Spinning Step

Undrawn fibers 110 having a fineness of 2.95 dtex were melt-spun using a melt spinning apparatus including a spinneret 101, rollers 102 and 103, and a winding device 104 illustrated in FIG. 4A under conditions similar to those in Comparative Example 1.

(2) Drawing Step

The undrawn fibers 110 were drawn using a two-stage drawing apparatus in which a preliminary drawing bath 112 performing heating in warm water and a main drawing bath 114 performing heating with heated saturated vapor were arranged between three rollers 111, 113, and 115 illustrated in FIG. 4B, to obtain a drawn composite fiber 120. Specifically, the speed of the introduction roller 111 was set at 10 m/min, the speed of the preliminary drawing delivery roller 113 was set at 29 m/min, and a bundle (fiber bundle) in which the undrawn fibers 110 obtained in the spinning step were tied was subjected to preliminary drawing treatment in warm water at 93° C. in the preliminary drawing bath 112. Subsequently, the speed of the drawn fiber delivery roller 115 was increased, main drawing was performed in pressurization saturated vapor at 124° C. in the main drawing bath 114, and the obtained drawn composite fiber 120 was wound by a winder 116.

As a result, the speed of the drawn fiber delivery roller 115 and a draw magnification, at which yarn breakage did not occur in a spinning step and a drawing step, and it was possible to perform industrially stable drawing, were 80 m/min and 8.0 times, respectively. Moreover, the fineness of a drawn composite fiber of Comparative Example 3 produced under such conditions was 0.4 dtex.

Comparative Example 4

An undrawn fiber having a fineness of 2.95 dtex was melt-spun by a method and under conditions similar to those in Comparative Example 3 except that the core material A and the sheath material a were used. The undrawn fiber was drawn, in a step other than the spinning step, by a method and under conditions similar to those in Comparative Example 3.

As a result, the speed of the drawn fiber delivery roller 115 and a draw magnification, at which yarn breakage did not occur in a spinning step and a drawing step, and it was possible to perform industrially stable drawing, were 80 m/min and 8.0 times, respectively. Moreover, the fineness of a drawn composite fiber of Comparative Example 4 produced under such conditions was 0.4 dtex.

Comparative Example 5

An undrawn fiber having a fineness of 3.98 dtex was melt-spun by a method and under conditions similar to those in Comparative Example 4 except that the rotation number of a gear pump was adjusted as appropriate so that an intended fineness was achieved. The undrawn fiber was drawn, in a step other than the spinning step, by a method and under conditions similar to those in Comparative Example 3.

As a result, the speed of the drawn fiber delivery roller 115 and a draw magnification, at which yarn breakage did not occur in a spinning step and a drawing step, and it was possible to perform industrially stable drawing, were 54 m/min and 5.4 times, respectively. Moreover, the fineness of a drawn composite fiber of Comparative Example 5 produced under such conditions was 0.8 dtex.

Comparative Example 6

(1) Spinning Step

An undrawn fiber having a fineness of 1.88 dtex was melt-spun by a method and under conditions similar to those in Example 1 except that a sheath-core ratio was set at 35/65.

(2) Drawing Step

Undrawn fibers were drawn in a step other than the spinning step using a drawing apparatus in which a warm water drawing bath was arranged between two rollers. Specifically, a bundle (fiber bundle) in which the undrawn fibers obtained in the spinning step were tied was subjected to drawing treatment in warm water at 93° C. in the warm water drawing bath under conditions of an introduction roller speed of 10 m/min and a drawn fiber delivery roller speed of 51 m/min.

As a result, yarn breakage occurred in the drawing step at a drawn fiber delivery roller speed of 51 m/min, and it was impossible to obtain a drawn composite fiber at a draw magnification set at 5.1 times.

The evaluation results of the drawn composite fibers of Examples and Comparative Examples, produced by the methods described above, are set forth in Tables 1 and 2 described below.

TABLE 1 Exam- Exam- Exam- Exam- Exam ple 1 ple 2 ple 3 ple 4 ple 5 Produc- Sheath-core 35/65 25/75 50/50 50/50 50/50 tion ratio [sheath condi- material/core tions material] Core material 20 18 20 30 30 MFR (g/10 min) Sheath 40 40 40 40 70 material MFR (g/10 min) Core material 0.5 0.45 0.50 0.75 0.43 MFR/sheath material MFR Spinning Consec- Consec- Consec- Consec- Consec- drawing utive utive utive utive utive method Fineness 1.88 1.72 1.60 0.80 0.80 of undrawn fiber (dtex) Draw 5.10 4.67 4.34 4.34 4.34 magnification (times) Eval- Fineness 0.4 0.4 0.4 0.2 0.2 uation of drawn composite fiber (dtex) Single yarn 7 7 6 7 6 maximum strength (cN/dtex) Single yarn 75 90 70 75 73 elastic modulus (cN/dtex) Bundle 6.6 7.1 6.8 6 to 7 7.5 thermal shrinkage (%) [120° C.]

TABLE 2 Compar- Compar- Compar- Compar- Compar- Compar- ative ative ative ative ative ative Exam- Exam- Exam- Exam- Exam- Exam- ple 1 ple 2 ple 3 ple 4 ple 5 ple 6 Produc- Sheath-core 50/50 60/40 50/50 50/50 50/50 35/65 tion ratio [sheath condi- material/core tions material] Core material 60 20 60 20 20 20 MFR (g/10 min) Sheath material 70 40 70 40 40 40 MFR (g/10 min) Core material 0.86 0.50 0.86 0.50 0.50 0.50 MFR/sheath material MFR Spinning Consec- Consec- Inconsec- Inconsec- Inconsec- Inconsec- drawing utive utive utive utive utive utive method Fineness of 1.60 1.60 2.95 2.95 3.98 1.88 undrawn fiber (dtex) Draw 4.34 4.34 8.0 8.0 5.4 5.1 magnification (set (times) value) Eval- Fineness 0.4 0.4 0.4 0.4 0.8 — uation of drawn composite fiber (dtex) Single yarn 4 5 5 to 6 7 5 to 6 — maximum strength (cN/dtex) Single yam 50 60 60 80 50 — elastic modulus (cN/dtex) Bundle thermal 9.2 9 5 10 5 — shrinkage (%) [120° C.]

As set forth in Table 2 above, the drawn composite fibers of Comparative Examples 1 and 3, in which resins having MFRs of more than 30 g/10 min were used in the core materials, had low single yarn strengths and low elastic moduli. The drawn composite fiber of Comparative Example 2, having a sheath-core ratio (sheath material/core material) of 60/40 and a small content of core material, had a low single yarn strength and a low elastic modulus. The drawn composite fiber of Comparative Example 4, in which the spinning step and the drawing step were performed as separate steps, was drawn at a high magnification of 8 times, was able to result in enhancement of a single yarn strength and an elastic modulus, and also resulted in an increase in bundle thermal shrinkage.

In contrast, the drawn composite fiber of Comparative Example 5, in which the spinning step and the drawing step were performed as separate steps, and a low draw magnification was further set at 5.4 times, resulted in prevention of an increase in bundle thermal shrinkage but resulted in a low single yarn strength and a low elastic modulus. In Comparative Example 6, in which an undrawn fiber of which the fineness was equal to that in Example 1 was spun in a step other than the drawing step, and the undrawn fiber was subjected only to warm water drawing without preliminary drawing, yarn breakage occurred in the drawing step before achieving at a needed draw magnification, and it was impossible to produce a fiber for evaluation.

In contrast, the drawn composite fibers of Examples 1 to 5, produced in the scope of the present invention, had a bundle thermal shrinkage of 8% or less at 120° C. and a single yarn strength of 6 cN/dtex or more although having a fineness of 0.6 dtex or less, as set forth in Table 1 above.

On the basis of the results, it was confirmed that a drawn composite fiber having a fineness in a range of 0.6 dtex or less, a low thermal shrinkage, and a high single yarn strength is obtained according to the present invention. 

1. A drawn composite fiber comprising a sheath-core structure in which a resin containing a crystalline propylene-based polymer as a main component is a core material, and a resin containing, as a main component, an olefinic polymer of which a melting point is lower than that of the core material is a sheath material, wherein the drawn composite fiber has a fineness of 0.6 dtex or less, a melt flow rate of the core material at a load of 21.18 N at 230° C. is 10 to 30 g/10 min, a ratio between cross-sectional areas of the sheath material and the core material (sheath material/core material) is 50/50 to 10/90, and the drawn composite fiber has a single yarn elastic modulus of 70 cN/dtex or more.
 2. The drawn composite fiber according to claim 1, wherein a ratio between a melt flow rate of the core material at a load of 21.18 N at 230° C. and a melt flow rate of the sheath material at a load of 21.18 N at 230° C. (core material/sheath material) is 0.3 to
 1. 3. A non-woven fabric formed using the drawn composite fiber according to claim
 1. 4. A method of producing a drawn composite fiber, comprising: a spinning step of obtaining, by melt-spinning, an undrawn fiber including a sheath-core structure in which a resin containing a crystalline propylene-based polymer as a main component is a core material, and a resin containing, as a main component, an olefinic polymer of which a melting point is lower than that of the core material is a sheath material; and a drawing step of obtaining a drawn composite fiber of 0.6 dtex or less by drawing treatment of the undrawn fiber, wherein the undrawn fiber has a fineness of 4.0 dtex or less, and has a ratio between cross-sectional areas of the sheath material and the core material (sheath material/core material) of 50/50 to 10/90, the core material has a melt flow rate of 10 to 30 g/10 min at a load of 21.18 N at 230° C., and the spinning step and the drawing step are consecutively performed.
 5. The method of producing a drawn composite fiber according to claim 4, wherein a ratio between a melt flow rate of the core material at a load of 21.18 N at 230° C. and a melt flow rate of the sheath material at a load of 21.18 N at 230° C. (core material/sheath material) is 0.3 to
 1. 6. The method of producing a drawn composite fiber according to claim 4, wherein the undrawn fiber is drawn at a draw magnification of 2 to 7 times in the drawing step.
 7. The method of producing a drawn composite fiber according to claim 5, wherein the undrawn fiber is drawn at a draw magnification of 2 to 7 times in the drawing step.
 8. The non-woven fabric according to claim 3, wherein a ratio between a melt flow rate of the core material at a load of 21.18 N at 230° C. and a melt flow rate of the sheath material at a load of 21.18 N at 230° C. (core material/sheath material) is 0.3 to
 1. 